1. Field of the Invention
The present invention relates to a fuel cell power generation system and a method of controlling the same, and more particularly to an apparatus for controlling fuel cells to be driven when at least a part of the fuel cell subsystems are out of order or in an abnormal state upon operation in a low power generation mode. The invention also relates to a method of controlling fuel cells.
2. Description of the Prior Art
Fuel cells are power generating devices which directly convert the chemical energy of fuels into electric energy. Each fuel cell has an electrolyte layer sandwiched by a pair of electrodes to which there are fed respectively an oxidant gas and a fuel gas from a reformer. The gases are reacted electrochemically in each electrode catalyst layer to generate electricity which is transmitted to outside the system.
Fuel cell power generation system comprises a large volume plant which includes a plurality of fuel cell stacks, referred to simply as stacks, connected to each other in series or in parallel. Each stack in turn has a plurality of unit cells connected in series.
In such a large volume fuel cell power generation system, the more the number of stacks the more the volume of the system and at the same time the more the frequency at which the stacks would encounter trouble. In conventional arrangements, operation of the whole power generation system must be stopped even when only one of the stacks is out of order, and operation of the system is not started again until the stack which is out of order has been recovered.
However, stopping operation of the whole system is undesirable for commercial scale power generation. Hence, it would be natural, in order to prevent stoppage of the whole power generation plant, to disconnect the or each gas conduit and the or each electric conduit of the disturbed stack or stacks from the power generation plant and continue operation of the power generation system by using only those remaining stacks that are functioning normally. When this is contemplated, however, it is difficult for the power generation system to continue to operate in some instances since the reformer, the air blower serving as an oxidant gas feeder, etc. have together a minimum load for operation. The power generation system will no longer operate if it is operated so that its output does not exceed the minimum load of the reformer and the air blower taken together when one or more stacks are out of order; i.e., if it is operated in a so-called lower output operation mode.
It would be possible to avoid stoppage of the whole power generation system by providing a plurality of reformers and air blowers each having a small volume, and controlling the number of reformers and air blowers to be used once trouble occurs to decrease the minimum load on the system. However, this solution is impractical since it leads to an increase in the amount of equipment required, an increased area for the arrangement, a decrease in the efficiency of the devices, and a complicated control apparatus.